The present invention generally relates to apparatus and method for photolithography and more particularly, relates to apparatus and method for immersion lithography wherein a fluid fills a gap formed between an imaging lens and a wafer surface.
The resolution of optical projections imaging is proportional to the imaging wavelength xcex and inversely proportional to the numerical aperture (NA) of the imaging lens. The numerical aperture is the product of the refractive index n of the light propagation medium between the imaging lens and the image and sine of the half aperture angle of the imaging lens sinxcex8. Presently, xcex is reduced to 193 nm and development work is ongoing for 153 nm while sinxcex8 is approaching 0.9 in many roadmaps. To further reduce xcex calls for vacuum and reflective optical systems such as in the case of 13.4 nm EUV imaging. Immersion lithography, which employs a high refractive index fluid between the last surface of the imaging device and the first surface on a wafer or substrate, offers a means to increase the numerical aperture and to reduce the wavelength without concern of the physical limitations.
Immersion microlithography is a known technique for improving the resolution in optical microscope. A drop of high index fluid is placed between the front surface of the microscopic objective lens and the observed sample. Immersion lithography also uses a high index fluid between the front surface (or the last surface) of the imaging lens and the first surface on a wafer or substrate. However, simply putting a drop of fluid between these two surfaces is not sufficient for modern projection mask aligner. Many manufacturing problems have to be overcome.
It is therefore an object of the present invention to provide an apparatus for immersion lithography that does not have the drawbacks or shortcomings of the conventional photolithography apparatus.
It is another object of the present invention to provide an apparatus for immersion lithography that does not have the photoresist outgassing problem.
It is a further object of the present invention to provide an apparatus for immersion lithography that does not have the wafer overheating problem.
It is another further object of the present invention to provide an apparatus for immersion lithography that does not have the particle contamination problem.
It is still another object of the present invention to provide an apparatus for immersion lithography that includes an imaging lens, a wafer and a fluid filling a gap formed between the imaging lens and the wafer.
It is yet another object of the present invention to provide an apparatus for immersion lithography wherein a fluid having a refractive index between about 1.0 and about 2.0 is used to fill a gap formed in-between an imaging lens and a wafer.
It is still another further object of the present invention to provide a method for conducting immersion lithography by flowing a fluid through a gap formed in-between an imaging lens and a wafer.
In accordance with the present invention, an apparatus and a method for immersion lithography are provided.
In a preferred embodiment, an apparatus for immersion lithography is provided which includes an imaging lens that has a first surface; a wafer that has a top surface to be exposed positioned spaced-apart and juxtaposed to the front surface of the imaging lens; and a fluid that has a refractive index between about 1.0 and about 2.0 fill a gap formed between the front surface of the imaging lens and the top surface of the wafer.
The apparatus for immersion lithography further includes a wafer holding device for holding the wafer in place, and a fluid-retaining means for maintaining the fluid in the gap formed in-between the front surface of the imaging lens and the top surface of the wafer. The apparatus further includes cover means for sealing the fluid around the imaging lens. The fluid-retaining means may further include fluid circulating means for circulating the fluid through the gas formed in-between the front surface of imaging lens and the top surface of the wafer; a temperature controlling means for controlling a temperature of the fluid; and a fluid-filter means for substantially maintaining said fluid particle-free.
In the apparatus for immersion lithography, the fluid circulating means may be a pump device. The fluid-retaining means may further include a fluid inlet for replenishing fluid in the fluid-retaining means; and a fluid outlet for discharging fluid from the fluid-retaining means. The wafer holding means may be a vacuum means or a mechanical means. The wafer holding means may further include wafer-tilting means. The apparatus may further include mirror means for monitoring a position of the imaging lens and the wafer.
The apparatus for immersion lithography may be a scan-and-repeat mask aligner, a step-and-repeat mask aligner or a proximity mask aligner.
The present invention is further directed to a method for conducting immersion lithography which can be carried out by the operating steps of first providing an imaging lens that has a front surface; then positioning a wafer that has a top surface to be exposed spaced-apart and juxtaposed to the front surface of the imaging lens; and maintaining a fluid that has a refractive index between about 1.0 and about 2.0 in-between the front surface of imaging lens and the top surface of the wafer.
The method for conducting immersion lithography may further include the step of moving the wafer in a lateral direction for repeated exposures, or the step of moving the imaging lens in a longitudinal directions for focusing. The method may further include the step of submerging the wafer in the fluid, or the step of controlling a flow of the fluid between the front surface of the imaging lens and the top surface of the wafer, or the step of filtering the fluid to substantially remove all particles. The method may further include the step of controlling a temperature of the fluid.